Heat capacity and vapor-pressure measurements indicate a higher-order phase transition in nitrogen monolayers adsorbed on basal-plane graphite. The transition coincides with finite jumps in three experimental quantities: the two-dimensional compressibility, the isosteric heat of adsorption, and the heat capacity at constant area. Thermodynamics implies that the heat capacity at constant two-dimensional spreading pressure also has an abrupt jump and hence the transition is suggestive of a second-order Ehrenfest type.

• Received 24 July 1975

DOI:https://doi.org/10.1103/PhysRevLett.35.1718